The present invention relates to the peroxidation of secondary carbon in alkanes, including alkyl groups attached to aromatic rings and cycloalkanes.
It is well known that the various tertiary and secondary organic hydroperoxides are suitable as oxidants for a number of highly selective partial oxidations. Molecular oxygen is usually not very suitable for these oxidations since a selective or partial oxidation requires, in a storchrometric sense, the transfer of one oxygen atom only. For example, organic hydroperoxides are particularly effective for the synthesis of alcohols, aldehydes, ketones, epoxides, cyanates and oximes.
Typical industrial processes for oxidizing alkanes and cycloalkanes involve oxidation in the presence of a catalyst such as a cobalt-naphthanate catalyst. The peroxide which is formed is only transitory and is subject to decomposition in the presence of the catalyst. The primary products are the corresponding alcohol and ketone. For example, the most widely used oxidation process for cyclohexane has only about 4 to 5% conversion of the cyclohexane and the product is about 75% cyclohexanol and cyclohexanone with the remainder being various by-products. At higher conversion levels, the selectivity of products decreases because the cyclohexanone is readily oxidized to by-products. The solution is to minimize the formation of the cyclohexanone by keeping the conversion of cyclohexane at a low level or preventing the formation of cyclohexanone from cyclohexanol such as by esterification. None of these processes produce any significant quantity of the peroxide, cyclohexyl hydroperoxide, in the final product. Typically the amount is only 5 to 10% of the product.
The main objective of peroxidations is the production of the corresponding hydroperoxides. However, the decomposition of hydroperoxides is required to maintain the free radical type chain reaction. This represents a loss in selectivity for the desired hydroperoxide. In addition, the alcohols or ketones formed from the decomposition of secondary hydroperoxides are more readily oxidized than the corresponding hydrocarbons. This over-oxidation leads to the rupture of carbon-carbon bonds and to the formation of carboxylic acids, which also facilitate the decomposition of the hydroperoxides. The result is a rapid decrease in the selectivity of products with increasing conversion.
To avoid this yield loss, the oxidation/peroxidation of the secondary carbon in various alkanes and cycloalkanes has to be carried out without the usual transition metal ions, which catalyze the decomposition of hydroperoxides. Also, the reactions have to be stopped at relatively low conversion levels. The commercial use of secondary carbon oxidations is limited by these two problems. First, in a non-catalytic oxidation, the initiation of the chain reaction must rely on the thermal decomposition of some hydroperoxide. This means that at low temperature the reaction is very slow while at high temperature the selectivity is low. Secondly, to produce the hydroperoxide of the secondary carbon at high selectivity, one can not rely on its decomposition for supplying the free radicals.
U.S. Pat. No. 3,987,115 discloses a method for peroxidation with oxygen in the presence of a tertiary alcohol and a tertiary hydroperoxide. In this process, the minimum ratio of tertiary alcohol to the hydrocarbon is relatively high (0.05 to 1) and the minimum ratio of tertiary alcohol to tertiary hydroperoxide is also high (1.21 in Run 4 of Table 1) and it is stated that such a low level of tertiary alcohol is undesirable because of the drop in the yield. Although the tertiary alcohol, present in the reaction mixture will form esters with the acidic hydroperoxide and thereby stabilize the secondary hydroperoxide, the tertiary alcohol prevents the tertiary hydroperoxide from forming free radicals. This will reduce the rate and selectivity of the peroxidation reaction.